Methods and apparatus for varying the back-up rate for a ventilator

ABSTRACT

A ventilator device delivers ventilatory support to a patient in a back up timed mode when patient respiration is not detected or a spontaneous mode when patient respiration is detected. The timing threshold governing the back-up mode is chosen to deviate from normal expected respiration time for the patient to promote patient initiated ventilation in the spontaneous mode but permit back-up ventilation in the event of apnea. Automated adjustments to the timing threshold during the timed mode are made from the less vigilant timing threshold to a more vigilant threshold at or near a timing of normal expected breathing of the patient. Such adjustments may be made from a minimum to a maximum vigilance timing settings or incrementally there between as a function of time in the timed mode which is preferably the number of delivered machine breaths.

This application claims the benefit of Australian provisional patentapplication no. 2003901042, filed on Mar. 7, 2003.

FIELD OF THE INVENTION

The invention relates to the field of automatically controlledmechanical ventilators for use in treating respiratory disorders such asrespiratory insufficiency. In particular, the invention relates to amethod and apparatus providing a more appropriate back-up rate forventilation when patients are not breathing spontaneously.

BACKGROUND OF THE INVENTION

Both positive and negative pressure mechanical ventilators have beenused for decades to treat patients with respiratory disorders. A rangeof ventilators are described in “Principles & Practice of MechanicalVentilation”, Edited by M J Tobin (1994, McGrawHill Book Company, ISBN0-07-064943-X). Other ventilators are described in “Respiratory TherapyEquipment”, by S. P. McPherson (3rd Ed., 1985, C. V. Mosby Company, ISBN0-8016-3312-5). Other ventilators are described in “AutomaticVentilation of the Lungs” by Mushin et al (3rd Ed, 1980, BlackwellScientific Publications, ISBN 0-632-002286-7).

Positive pressure ventilators provide a supply of air or breathable gasat positive pressure to a patient's airway. Flow is volume of air perunit time. Tidal volume is the volume of air entering and leaving thelungs during the respiratory cycle. Minute ventilation is the volume ofair delivered to a patient in 1 minute. There are two general approachesto control of ventilators: (1) volume or flow; and (2) pressure control.A ventilator may be programmed to control the volume of air delivered toa patient by adjusting the minute ventilation. The rate at which the airis delivered to the patient is breaths (or cycles) per unit time.

In order to achieve the desired minute ventilation, both the rate andvolume of air delivered to a patient can be varied.

In this specification, a ventilator will be said to be triggered into aninspiratory phase and cycled into an expiratory phase. Spontaneousbreaths are those that are initiated by the patient. If the ventilatordetermines either the start or end of inspiration, then the breath isconsidered mandatory. If the patient triggers the ventilator (e.g., witha spontaneous breath), the ventilator is said to be an assistor. If timetriggers the ventilator into the inspiratory phase, the ventilator issaid to be a controller. If the patient can assist and the machine canback him up (if his breathing rate drops or stops altogether), theventilator is designated an assistor/controller. It is possible for amachine to be all three. It is:

-   -   (1) an assistor when it is patient-triggered and there is no        timed backup rate;    -   (2) a controller when it is time-triggered and no assist        mechanism is provided; or    -   (3) an assistor/controller when the timed rate backs up the        patient's rate (sometimes called “spontaneous/timed”).

When the ventilator switches between inspiratory and expiratory modes atthe same time as the spontaneously breathing patient, the ventilator issaid to be in synchrony with the patient. Loss of synchrony can lead topatient discomfort and ineffective ventilation. For purposes of thisdescription, a spontaneous/timed ventilator is considered to be in aspontaneous mode when it is delivering ventilation support in responseto detected patient respiration. Similarly, the spontaneous/timedventilator is considered to be in a timed mode when it is delivering amachine breath according to a back up timing threshold back up rate inresponse to a failure to detect patient respiration.

A method for providing ventilatory assistance in a spontaneouslybreathing subject is described in U.S. Pat. No. 6,484,719(Berthon-Jones), the contents of which are hereby incorporated bycross-reference.

In some situations, a spontaneous/timed ventilator can fail to detectwhen the patient switches between inspiration and expiration. Thereforesome ventilators have a “time-out” for a spontaneous mode. Suchventilators will switch from the spontaneous mode (waiting for thepatient) to a timed mode (delivering ventilation at the back up rate) atthe end of the time-out period. An improved system for a “time-out” isdescribed in U.S. Pat. No. 6,213,119, the contents of which are herebyincorporated by cross-reference.

As discussed herein, the back-up rate (cycles/time) may be alternativelydescribed by its reciprocal, the back-up period (time/cycle).

In programming an automatic ventilator, the problem arises as to thechoice of the most appropriate back-up rate for the device.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, a ventilator isdesigned with a back-up process or back up timing module such that it isconfigured or programmed to deliver timed breaths with a timingthreshold or back-up rate in the event a spontaneous breath is notdetected. The timing threshold or back up rate may, in a firstembodiment, be set to a first rate that is substantially lower (i.e.,less frequent) than the patient's normal respiratory rate. In thisregard it is chosen to depart or deviate from that which wouldordinarily be associated with normal or expected patient respiration ina manner that promotes patient initiated ventilation by the ventilatorbut allows timed ventilation in the event of apnea. Accordingly, thefirst rate is set substantially lower so that the ventilator will lesslikely interfere with the patient's normal breathing cycle, meaning thatthe chances for the ventilator to resume a spontaneous mode from a timedmode or continue within spontaneous mode are increased when theinstantaneous backup rate is lower than the backup rate which would havebeen set with a conventional fixed backup rate ventilator. In otherwords, if the patient again commences spontaneous breathing, the timedmode is discontinued or the spontaneous breathing mode continues. If thepatient continues to require timed ventilation in the absence ofdetected breathing, meaning that spontaneous breathing has notre-commenced, the ventilator will automatically normalize the timingthreshold to a more vigilant timing or an approximately normal expectedrespiratory timing. Thus, the ventilator gradually or step-wiseincreases the breathing rate from the first rate to a second rate whichis at, closer to, or slightly lower than the patient's normal breathingrate. Of course, the second rate may also change to be higher than thepatient's normal breathing rate, depending on the particularapplication.

In other words, a low timed rate is used to decrease the likelihood offalse entry into the timed mode of the ventilator yet the timing rate isadjustable in the timed mode to permit appropriate ventilation of apatient during the timed mode. Thus, the back-up rate is automaticallyadjusted as a function of elapsed time in the timed mode, increasingtowards the patient usual or average respiratory rate. In terms of atiming threshold that is a back-up period, the back-up period isautomatically adjusted as a function of the time in the timed mode,decreasing towards the patient's usual or average respiratory period.

In another aspect, the timed rate may be adjustable between two limits,namely, a first lower rate and a second rate, higher than the first. Inone form, the first rate is significantly lower than the patient's usualrespiratory rate and the second rate is at or slightly lower than thepatient's usual respiratory rate. Of course, the second higher rate mayoptionally be at or higher than the patient's usual or averagerespiratory rate. When the ventilator switches to the timed mode, havingfailed to detect a spontaneous breath, the ventilator is initially setto the first lower rate and then the patient is ventilated. If after apredetermined period, a spontaneous patient breath still has not beendetected by the ventilator, the back-up rate is increased towards thesecond rate. In one preferred form, apparatus in accordance with theinvention changes the back-up rate from the lower rate to the fasterrate over approximately 5 breaths within the timed mode if spontaneousbreaths are not detected.

In one form, in apparatus in accordance with the invention, the secondrate is approximately 25% faster than the first rate. In anotherpreferred form, the second rate is approximately 50% faster than thefirst rate.

In another embodiment, the adjustable back-up rate is automaticallymodified as a function of adequacy of ventilation. In a preferredembodiment of such an adjusting back-up rate, ventilation adequacyaffects the rate of change of the adjustments to the back-up ratebetween its minimum and maximum values.

In one embodiment, within the timed mode the device periodically returnsthe timing threshold to a less vigilant timing threshold to promoteresynchronization with the patient.

Other aspects of the invention are described in the detailed descriptionherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a ventilator programmed in accordance with an embodiment ofthe invention;

FIG. 2 is a flow chart of one embodiment of steps taken in the automaticadjustment of a timing threshold;

FIG. 3 is a flow chart of an alternative embodiment of steps taken inthe automatic adjustment of a timing threshold;

FIG. 4 shows a “delta” function for adjusting the back-up rate inaccordance with the adequacy of ventilation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows, by way of example, apparatus suitable for performing theinvention. FIG. 1 shows an impeller 1 connected to an electric motor 2under the control of a servo-controller 3 which is in turn under thecontrol of a controller 4. In one form the controller 4 is amicro-processor based controller, such as an Intel '486 microprocessor.The impeller 1 and motor 2 form a blower. Air from the blower passesalong a flexible conduit 6 to a patient interface such as a nasal mask 5with a vent 9. While a nasal mask is illustrated, the invention may beused in conjunction with a nose-and-mouth mask, full face mask orendo-tracheal tube. A number of switches 7 are connected to thecontroller. A number of sensors are connected to the controller, namely:flow 10, pressure 11, snore 12, motor speed 13 and motor current 14.There are a set of displays 8 connected to the controller 4 fordisplaying information from the controller. There is an interface 15 toenable the controller 4 to communicate with an external device such as acomputer. With such a device, changes in the speed of the blower may becontrolled to alternatively change the pressure in the mask to implementventilatory support. Optionally, the blower motor speed may be heldgenerally constant and pressure changes in the mask may be implementedby controlling an opening of a servo-valve (not shown) that may variablydivert/vent or deliver airflow to the mask. Those skilled in the artwill recognize other devices for generating ventilatory support anddelivering same to a patient.

The controller 4 or processor is configured and adapted to implement themethodology described herein and may include integrated chips, a memoryand/or other instruction or data storage medium. For example, programmedinstructions with the control methodology may be coded on integratedchips in the memory of the device or such instructions may be loaded assoftware. With such a controller, the apparatus can be used for manydifferent pressure ventilation therapies simply by adjusting thepressure delivery equation that is used to set the speed of the bloweror to manipulate the venting with the release valve. Those skilled inthe art will also recognize that aspects of the controller may also beimplemented by analog devices or other electrical circuits.

Generally speaking, as illustrated by the step of the flowchart of FIG.2, a device in accordance with the invention can deliver ventilatorysupport in a spontaneous mode (step 22) to synchronously ventilate thepatient in accordance with detecting the patient's inspiration (step20). However, if the patient inspiration is not detected before thelapsing of some timing threshold (e.g., a timing rate (1/T_(current)) ortiming period (T_(current))) determined by comparison of the thresholdby a measured rate or period (step 24), the device will initiate a timedmode (step 26) in which a machine initiated breath will be delivered.Those skilled in the art will recognize methods for entering the timedmode by enforcing a timing threshold. Automated adjustments to thetiming threshold, (1/T_(current) or T_(current)) may then be implementedin steps 28 and 30 as described further herein.

In the following description, “period” means the reciprocal of rateunless otherwise indicated. Define T_(apn) to be a desired time for thebackup period while the patient is not breathing or the desired timefrom the start of an inspiratory breath (non-patient triggered) to thetime for the start of a subsequent inspiratory breath (non-patienttriggered), namely the reciprocal of the rate desired when the patientis not triggering the ventilator and is at least apparently, if notactually apnoeic, from the point of view of the ventilator. Preferablythis rate is approximately that of the normal expected respiratory ratefor the patient but it may be higher or lower as desired. DefineT_(spont) to be the desired time for the backup period while the patientis spontaneously breathing which is preferably chosen to deviate fromthe normal expected respiratory rate of the patient such that the timingthreshold is less vigilant than normal breathing. Define T_(current) tobe a current backup period being applied to control the delivery ofventilation to the patient.

If the current breath started at time t_(breathstart) then a machineinitiated or “timed” breath will occur at t_(breathstart)+T_(current) ifthe patient has not triggered the ventilator before this time. It is tobe understood that T_(spont) and T_(apn) are chosen so that with therespiratory mechanics of the Patient being treated, a decrease inrespiratory period from T_(spont) to T_(apn), with fixed pressuresupport levels, results in a monotonic increase in ventilation. Thoseskilled in the art will recognize various methods for determining thetime of recurring respiratory event t_(breathstart). or otherwisedetecting an event in the respiratory cycle for purposes of timing thedelivery of spontaneous or timed cyclical ventilation support.

(I) Basic Methodology

T_(current) is initialized to T_(spont). If a timed breath occurs,T_(current) is decremented by some amount Δ_(d)T_(current) (notnecessarily a constant) and the result is then limited to be at leastT_(apn). More specifically, in one implementation, T_(current) isdecremented by (T_(spont)−T_(apn))/N_(dec), where N_(dec) is, forexample, 5, so that in this case after 5 timed breaths the apnoeicbackup rate is reached. This adjustment to the timing threshold isillustrated in step 28 of FIG. 2.

Generally N_(dec) should not be too small (e.g., 1) because one wants tomaximize the chances of resynchronising with the patient. The longer theperiod of time during which the patient has not triggered theventilator, the less likely it is that triggering will occur in theimmediate future, and the more likely it is that hypoxia will occur iftimed ventilation continues at a low rate.

If a triggered breath occurs at any time, T_(current) is incremented bysome amount Δ_(i)T_(current), and the result is then limited so that itis at most T_(spont). More specifically, in one implementation,T_(current) is incremented by (T_(spont)−T_(apn))/N_(inc), where in thepreferred implementation N_(inc)=1, so that in this case after 1triggered breath the backup rate returns immediately to the spontaneousbreathing backup rate T_(spont). It is desirable that N_(inc)<N_(dec),because if one spontaneous breath is detected there are likely to bemore, and one wants to maximize the chance of triggering on the nextone. This adjustment to the timing threshold is illustrated in step 30of FIG. 2.

In one implementation as illustrated by FIG. 3, when successive timedbreaths are delivered with a period of T_(apn) occasional breaths (forexample, 1 breath in each contiguous series of 10 timed breaths) aredelivered with a longer period, up to T_(spont) in order to increase thechance of resynchronizing with the patient, while on average ventilatingthe patient at a rate close to the reciprocal of T_(apn). Thus, whilethe ventilator is in the timed mode, the back-up rate may beperiodically modified or relaxed to be less vigilant for the purpose ofpromoting resynchronization with the patient as illustrated in step 34of FIG. 3 if a certain number of consecutive timed breaths occur (step32). Thus, in the timed mode the rate or period may be decreased orincreased respectively for a breath cycle. For the following breathcycle the back-up rate is returned to a more vigilant rate or period(e.g., the patient normal expected or average breath rate or period) ifno patient breath is detected that would trigger the spontaneous mode asa result of the more relaxed back-up rate or period. For example, instep 30 the ventilator may return T_(current) to T_(spont) and thenrepeat the methodology above for increasing back toward T_(apn) or aspreferred, after a single machine generated breath, the ventilator willadjust T_(current) back to T_(apn) in step 28.

(II) Advantages

Advantages of the invention include the ability to set a lower backuprate than usual, so as to interfere less than usual with the patient'sspontaneous breathing, while setting a higher and thus more efficientrate which will prevail when the patient is genuinely or effectivelyapnoeic, thus allowing lower pressure support levels.

(III) Algorithm Using Measured Ventilation to Modify Back-Up RateVariation Speed

If a rapidly-responding measure of minute ventilation is available (witha response time typically of the order of 2 or 3 breaths, e.g., a 4^(th)order Bessel lowpass filter of the absolute value of flow with a cornerfrequency of about 3.2/60 Hz), and there is also available a desired ortarget ventilation (as in a servoventilator, the typical case envisagedhere, though the ventilator need not actually be servo-controlling theventilation—some advantages still accrue without servo-control), thenΔ_(d)T_(current) can be made to depend on some measure of the adequacyof the actual ventilation, for example, the ratio R_(vent) of themeasured ventilation to the target ventilation (e.g., V_(meas)/V_(targ))which may be determined from a signal from a flow sensor or differentialpressure transducer configured to do so. When R_(vent) is significantlylarger than 1, say ≧1.2, then Δ_(d)T_(current) can be chosen to be 0,because ventilation is entirely adequate. In the case of aservoventilator with a low or zero minimum pressure support level, ifall breaths are timed, R_(vent) cannot stay at ≧1.2 for any significantperiod of time, because the fact that R_(vent) is more than 1 will causethe pressure support level to be reduced so that R_(vent)≈1. Thesituation of a timed breath occurring with R_(vent) being ≧1.2 maytypically occur when a sigh is followed by a brief pause, and it is notdesirable to decrement T_(current) under these circumstances, because aspontaneous breath will probably occur very soon, and one wants tomaximize the chances of synchronizing with it. In the absence ofservo-control of pressure support level, R_(vent) being ≧1.2 indicatesthat more than adequate ventilation is being achieved at a low rate atthe set pressure level (presumably regarded as acceptable), so there isno need to increase the rate.

When R_(vent) is significantly less than 1, Δ_(d)T_(current) can beincreased, so that, for example, if there is marked hypoventilation,T_(current) may decrease from T_(spont) to T_(apn) in 2 breaths, becausein this situation it is desirable to get to the more efficient apnoeicbackup rate quite rapidly. Intermediate values of R_(vent) shouldproduce intermediate values of T_(current) in a monotonic fashion butnot necessarily using linear interpolation between the endpoints (i.e.,T_(spont) and T_(apn)). To derive the full benefit of this invention inthe case of a servoventilator, it is essential that Δ_(d)T_(current>)0when R_(vent)≈1. The reason for this is that the automatic increase inpressure support in response to the fact that when R_(vent)<1 it mayrapidly cause the hypoventilation to be corrected, i.e. R_(vent)≈1, sothat if Δ_(d)T_(current)≈0 when R_(vent)≈1, the backup period may neverdecrease, resulting in sustained ventilation of the patient at a lowerthan optimal rate using higher than optimal pressure. To prevent this,it is desirable that when R_(vent)≈1,Δ_(d)T_(current)≧(T_(spont)−T_(apn))/10.

FIG. 3 illustrates a piecewise linear “delta” function for adjusting theback-up rate in accordance with the adequacy of ventilation by thefollowing formula:Δ_(d) T _(current)=(T _(spont) −T _(apn))*K _(dec) (R _(vent))As depicted in the figure, the x-axis shows a measure of the adequacy ofventilation R_(vent). The y-axis shows the relative size of a correctionfactor (K_(dec)) to be applied to the back-up rate. For example, a valueof 0.2 on the x-axis indicates a low adequacy of ventilation which iscompensated by a relatively high correction factor, causing the back-uprate to be increased by a larger delta. In other embodiments of theinvention, the delta function may have different shapes, e.g., linear,curved and/or combinations thereof. The delta function may also haveother input parameters.

It should be noted that the combination of this algorithm with a fixedpressure level constitutes a form of servo-control of ventilation duringapnea, though this is not the primary intention of this invention. Sucha servo-control algorithm based on rate would require Δ_(d)T_(current)to be negative when the ventilation is above target.

In one form the apparatus has a learning mode as described in publishedAustralian patent application AU 24896/01 (Berthon-Jones) entitled“Determining Suitable Ventilator Settings in Patients with AlveolarHypoventilation During Sleep”, also disclosed in U.S. Pat. No.6,644,312, the disclosure of which is incorporated by reference. Duringthe learning mode, the device learns the patient's natural breathingrate. The lower back-up rate is then set as a function of the rate,preferably at approximately ⅔ or 67%, or alternatively 75%, of thatrate, and the higher rate is set at the rate determined to be thepatient's natural breathing rate or the average normal breathing ratedetermined from a period of time. Of course, the ventilator may includea function to prompt for input by a user of the higher and/or lowerrespiratory rates or periods as desired or necessary.

In one form the back-up rate is changed from the first rate to thesecond rate in a stepwise manner, by adding a “delta” to the back-uprate. The delta may be of fixed size, or it may be a function of ameasure of the adequacy of the ventilation. In the following example ofone embodiment of the invention, implemented in the C++ programminglanguage, the function determining the size of a delta function isillustrated:

int VPAP _ST _Sync_RelVentErrT::CalcBackupPeriodDecrement( ) {  //Return decrement in ticks at HZ.  int MaxBackupDecrement =BackupPeriodWhenBreathing -  BackupPeriodApnoeic;  double RelVentErr; if (RelativeVentilationErrGetP = NIL ||   !RelativeVentilationErrGetP−>Get ( RelVentErr )   ) // No informationavailable about ventilation error. return MaxBackupDecrement /5; // Agoal is that if the ventilator is providing timed breaths and Is // justreaching the target ventilation, we want to ensure that the // apnoeicrate is reached reasonably quickly. We don't want to get // stuckproviding high pressure support at a low rate and meeting // target thisway. if ( RelVentErr >= 0.3) // Well over target, so don't change backupperiod at all. This might // be e.g. a pause after a sigh.  return 0;double PropnOfMaxDecrement; if (RelVent Err >= −0.1) PropnOfMaxDecrement = 0.2*(0.3 − RelVentErr)/0.4; else  {   if(RelVentEff < −1.0)    RelVentErr = −1.0; // should be unnecessary  PropnOfMaxDecrement = (−0.1 − RelVentErr) / 0.9;   } return (int)(PropnOfMaxDecrement * MaxBackupDecrement + 0.5); }

Although the invention has been described with reference to preferredembodiments, it is to be understood that these embodiments are merelyillustrative of the application of the principles of the invention.Numerous modifications may be made therein and other arrangements may bedevised without departing from the spirit and scope of the invention.

1. A method for setting back up support ventilation for a patientcomprising the steps of: delivering pressure support ventilation to apatient during inspiratory and expiratory portions of a breathing cycleof the patient; detecting a recurrent respiratory event in a cycle ofthe patient's respiration; setting a timing threshold for controllingthe delivery of pressure support ventilation subsequent to the recurrentrespiratory event and in an absence of a detection of a subsequentrecurrent respiratory event; controlling a delivery of pressure supportaccording to the timing threshold in the absence of a detection of arecurrent respiratory event subsequent to a detected recurrentrespiratory event; wherein the step of setting a timing threshold, saidtiming threshold is chosen to deviate from a normal expected respirationtiming for the patient in a manner that promotes patient initiatedsynchronization of the pressure support ventilation but permits back-upventilation in the event of an apnea.
 2. The method of claim 1 furthercomprising the step of automatically adjusting the timing threshold inthe absence of a detection of a subsequent recurrent respiratory eventto normalize the timing threshold toward an expected normal respirationtiming for the patient.
 3. The method of claim 2 wherein the timingthreshold adjusts stepwise as a function of a number of delivered cyclesof ventilation delivered in response to the timing threshold.
 4. Themethod of claim 2 further comprising the step of automatically adjustingthe timing threshold in the presence of a detection of a subsequentrecurrent respiratory event to adjust the timing threshold towards atiming threshold chosen to deviate from a normal expected respirationtiming for the patient.
 5. The method of claim 4 wherein the timingthreshold adjusts stepwise as a function of a number of delivered cyclesof ventilation delivered in response to detected patient respiration. 6.The method of claim 5 wherein in response to a detection of a singlerespiratory cycle by the patient, the timing threshold returns to anoriginal timing threshold chosen to deviate from a normal expectedrespiration timing for the patient.
 7. The method of claim 6 wherein thetiming threshold is a respiratory rate, and in the step of automaticallyadjusting the timing threshold in the absence of a detection of asubsequent recurrent respiratory event, said respiratory rate increasesfrom a rate chosen to deviate from a normal expected respiration ratefor the patient toward a normal expected respiration rate.
 8. The methodof claim 7 wherein automated adjustments to the timing threshold arerestricted by a maximum respiratory rate that is about 50 percent fasterthan the rate chosen to deviate from a normal expected respiration rate.9. The method of claim 7 wherein automated adjustments to the timingthreshold are restricted by a minimum respiratory rate that iscalculated as a fraction of a normal respiratory rate determined duringa learning period.
 10. The method of claim 9 wherein the fraction isabout 67%.
 11. The method of claim 6 wherein the timing threshold is arespiratory period, and in the step of automatically adjusting thetiming threshold in the absence of a detection of a subsequent recurrentrespiratory event, said respiratory period decreases from a periodchosen to deviate from a normal expected respiration period for thepatient toward a normal expected respiration period.
 12. The method ofclaim 2 further comprising the step of determining adequacy of thepatient's ventilation and wherein automated adjustments to the timingthreshold are a function of the adequacy of the patient's ventilation.13. The method of claim 12 wherein the function of the adequacy of thepatient's ventilation includes a ratio of a measured ventilation and atarget ventilation.
 14. The method of claim 13 wherein the measuredventilation is a minute ventilation and the function is piecewise linearon a ratio of a measured minute ventilation to a target minuteventilation.
 15. The method of claim 2 further comprising the step ofautomatically periodically returning the normalized timing threshold tothe timing threshold chosen to deviate from a normal expectedrespiration in the absence of a detection of a subsequent recurrentrespiratory event.
 16. The method of claim 15 further comprising thestep of changing the timing threshold back towards the normalized timingthreshold after delivering a machine breath if a subsequent recurrentrespiratory event remains undetected during application of the timingthreshold chosen to deviate from a normal expected respiration.
 17. Themethod of claim 16 wherein the periodic return occurs as a function of anumber of delivered machine breaths.
 18. An apparatus for providing backup ventilation to a patient comprising: a ventilator adapted to generateand deliver ventilatory support to a patient; and a controllerconfigured to control the ventilatory support provided by theventilator, wherein the controller is configured with a spontaneous modein which synchronized cyclical ventilatory support is initiated bydetecting the presence of a respiratory cycle of the patient; whereinthe controller is further configured with a timed mode in which cyclicalventilatory support is delivered in accordance with a timing thresholdin the absence of detecting a respiratory cycle of the patient, andwherein the controller is configured to set the timing threshold as afunction of normal expected respiratory timing for the patient, saidfunction chosen to deviate the timing threshold from the normal expectedrespiration timing for the patient in a manner that promotes patientinitiated synchronization of the ventilatory support but permits back-upventilation in the event of an apnea.
 19. The apparatus of claim 18wherein the controller is further configured to automatically adjust thetiming threshold in a timed mode in the absence of detecting arespiratory cycle of the patient from a timing threshold deviating froma normal expected respiration timing for a patient toward a normalexpected respiration timing for the patient.
 20. The apparatus of claim19 wherein the timing threshold adjusts stepwise as a function of anumber of delivered cycles of ventilation delivered in response to thetiming threshold.
 21. The apparatus of claim 20 wherein the controlleris further configured to automatically change the timing threshold uponthe detection of the presence of a respiratory cycle of the patient byadjusting the timing threshold towards a timing threshold deviating froma normal expected respiration timing for the patient.
 22. The apparatusof claim 21 wherein the timing threshold adjusts stepwise as a functionof a number of delivered cycles of ventilation delivered in response todetected respiratory cycles of the patient.
 23. The apparatus of claim22 wherein in response to a detection of a single respiratory cycle bythe patient, the timing threshold returns to an original timingthreshold chosen to deviate from a normal expected respiration timingfor the patient.
 24. The apparatus of claim 19 wherein the timingthreshold is a respiratory rate, and the controller in automaticallyadjusting the timing threshold in the timed mode increases the timingthreshold from a minimum rate lower than a normal expected respirationrate for the patient toward a maximum higher rate approximately that ofnormal expected patient respiration.
 25. The apparatus of claim 24wherein the automated adjustments to the timing threshold are restrictedby a maximum respiratory rate that is about 50 percent faster than theminimum rate.
 26. The apparatus of claim 24 wherein the minimumrespiratory rate is calculated as a fraction of a normal respiratoryrate determined during a learning period.
 27. The apparatus of claim 26wherein the fraction is about 67%.
 28. The apparatus of claim 18 whereinthe function of normal expected respiratory rate for the patient is afixed percent of a learned respiratory rate for the patient.
 29. Theapparatus of claim 28 wherein the fixed percent is about 67%.
 30. Theapparatus of claim 19 wherein the timing threshold is a respiratoryperiod, and the controller in automatically adjusting the timingthreshold decreases it from a maximum period deviating from a normalexpected respiration period toward a minimum lower period approximatelythat of normal expected respiration.
 31. The apparatus of claim 18wherein the controller is further configured to determine adequacy ofthe patient's ventilation and adjust the timing threshold as a functionof the adequacy of the patient's ventilation.
 32. The apparatus of claim31 wherein the function of the adequacy of the patient's ventilationincludes a ratio of a measured ventilation and a target ventilation. 33.The apparatus of claim 32 wherein the measured ventilation is a minuteventilation and the function of the adequacy of the patient'sventilation is piecewise linear on a ratio of a measured minuteventilation to a target minute ventilation.
 34. The apparatus of claim18 wherein the controller is further configured in the absence of adetection of a respiratory cycle of the patient to automaticallyperiodically return the timing threshold from a more vigilant thresholdapproximating normal expected respiration timing to a less vigilantthreshold deviating from normal expected respiration timing.
 35. Theapparatus of claim 34 wherein the controller is further configured tochange the timing threshold to the more vigilant threshold from the lessvigilant timing threshold after delivering a machine breath if asubsequent respiratory cycle of the patient remains undetected duringapplication of the less vigilant timing threshold.
 36. The apparatus ofclaim 34 wherein the controller periodically returns the timingthreshold as a function of a number of consecutive delivered machinebreaths.
 37. An apparatus for providing back up ventilation to a patientcomprising: a patient interface and conduit coupled for delivering acontrolled supply of breathable gas to a patient; a controllable blowerdevice coupled to the conduit to generate the supply of breathable gas,the supply of breathable gas providing ventilatory support for thepatient; a pressure transducer configured to detect a measure of patientrespiration through the interface and generate a signal indicative ofthe measure; and a processor coupled with the blower device andtransducer to control the delivery of the supply of breathable gas tothe patient, wherein the processor includes coded instructions tocontrol a spontaneous mode in which synchronized cyclical ventilatorysupport is initiated by detecting the presence of a respiratory cycle ofthe patient from the signal from the pressure transducer; wherein thecontroller is further includes coded instructions to control a timedmode in which cyclical ventilatory support is delivered in accordancewith a timing threshold in the absence of detecting a respiratory cycleof the patient from the signal from the pressure transducer, and whereinthe processor further includes coded instructions to set the timingthreshold as a function of normal expected respiratory timing for thepatient, said function chosen to deviate the timing threshold from thenormal expected respiration timing for the patient in a manner thatpromotes patient initiated synchronization of the ventilatory supportbut permits back-up ventilation in the event of an apnea.
 38. Theapparatus of claim 37 wherein the processor further includes codedinstructions to automatically adjust the timing threshold in a timedmode in the absence of detecting a respiratory cycle of the patient froma timing threshold deviating from a normal expected respiration timingfor a patient toward a normal expected respiration timing for thepatient.
 39. The apparatus of claim 38 wherein the timing thresholdadjusts stepwise as a function of a number of delivered cycles ofventilation delivered in response to the timing threshold.
 40. Theapparatus of claim 39 wherein the processor further includes codedinstructions to automatically change the timing threshold upon thedetection of the presence of a respiratory cycle of the patient toadjust the timing threshold towards a timing threshold deviating from anormal expected respiration timing for the patient.
 41. The apparatus ofclaim 40 wherein the timing threshold adjusts stepwise as a function ofa number of delivered cycles of ventilation delivered in response todetected respiratory cycles of the patient.
 42. The apparatus of claim41 wherein in response to a detection of a single respiratory cycle bythe patient, the timing threshold returns to an original timingthreshold chosen to deviate from a normal expected respiration timingfor the patient.
 43. The apparatus of claim 38 wherein the timingthreshold is a respiratory rate, and the coded instructions of theprocessor control automatically adjusting of the timing threshold in thetimed mode to increase the timing threshold from a minimum rate lowerthan a normal expected respiration rate for the patient toward a maximumhigher rate approximately that of normal expected respiration.
 44. Theapparatus of claim 43 wherein automated adjustments to the timingthreshold are restricted by a maximum respiratory rate that is about 50percent faster than the minimum rate.
 45. The apparatus of claim 44wherein coded instructions of the processor calculate the minimumrespiratory rate as a fraction of a normal respiratory rate determinedduring a learning period.
 46. The apparatus of claim 45 wherein thefraction is about 67%.
 47. The apparatus of claim 37 wherein thefunction of normal expected respiratory rate for the patient is afraction of a learned respiratory rate for the patient.
 48. Theapparatus of claim 47 wherein the fraction is about 67%.
 49. Theapparatus of claim 38 wherein the timing threshold is a respiratoryperiod, and the coded instruction of the processor control automaticallyadjusting the timing threshold by decreases it from a maximum perioddeviating from a normal expected respiration period toward a minimumlower period approximately that of normal expected respiration.
 50. Theapparatus of claim 37 wherein the processor further includes codedinstructions to determine adequacy of the patient's ventilation from asignal from the pressure transducer and adjust the timing threshold as afunction of the determined adequacy of the patient's ventilation. 51.The apparatus of claim 50 wherein the function of the adequacy of thepatient's ventilation includes a ratio of a measured ventilation and atarget ventilation.
 52. The apparatus of claim 51 wherein the measuredventilation is a minute ventilation and the function of the adequacy ofthe patient's ventilation is piecewise linear on a ratio of a measuredminute ventilation to a target minute ventilation.
 53. The apparatus ofclaim 37 wherein the processor further includes coded instructions tocontrol in the absence of a detection of a respiratory cycle of thepatient an automatic periodic return of the timing threshold from a morevigilant threshold approximating normal expected respiration timing to aless vigilant threshold deviating from normal expected respirationtiming.
 54. The apparatus of claim 53 wherein the coded instructionsfurther control a change of the timing threshold to the more vigilantthreshold from the less vigilant timing threshold after delivering amachine breath if a subsequent respiratory cycle of the patient remainsundetected.
 55. The apparatus of claim 53 wherein the coded instructionscontrol the automatic periodic return of the timing threshold as afunction of a number of consecutive delivered machine breaths.